1. Field of the Invention
The present invention relates to a functional substrate provided with a color filter, which is used in a color liquid crystal display, in particular, a color ferroelectric liquid crystal display as a display device, or in a color image pickup unit as an input device.
2. Related Background Art
Hitherto well known are liquid crystal devices in which scanning electrodes and signal electrodes are formed in a matrix fashion and the spaces between the electrodes are filled with liquid-crystal compounds to form a large number of picture elements (pixels) so that an image or information can be displayed. Employed as a system of driving such display devices is time-sharing drive wherein address signals are selectively applied to the scanning electrodes in a successive and periodic fashion and given information signals are selectively applied to the signal electrodes in a synchronous and parallel fashion to the address signals.
In recent years, aiming at making larger the screen of liquid crystal display units, it is progressed to use the surface-stabilized ferroelectric liquid crystal devices disclosed, for example, in U.S. Pat. No. 4,367,924 and No. 4,638,088. As the screen is made larger, the scanning electrode and signal electrode of the matrix electrode have become increasingly long, so that the effect of delaying an applied voltage (or applied-voltage delay effect) has brought about a serious problem.
Conventional TN (twisted nematic) liquid crystal devices or STN (super twisted nematic) liquid crystal devices employ multiplexing drive in which the applied voltage is periodically applied (namely, a single picture with a high contrast is formed by plural-frame scanning), and hence the lowering of display quality level, ascribable to the applied-voltage delay effect mentioned above, has been little questioned. In the instance of the ferroelectric liquid crystal device, however, a single picture with a high contrast must be formed by single-frame scanning, so that the applied-voltage delay effect mentioned above has brought about a serious problem. Heat generation accompanying wiring resistance is also incidental to such a delay effect to cause a non-uniformity of temperature distribution in cells, resulting also in a lowering of display quality level.
For the foregoing reasons, in applying the ferroelectric liquid crystal device to a large screen display panel, a method has been employed in which a metallic film or an alloy film is wired in contact in the longitudinal direction of the scanning electrode and signal electrode so that the applied-voltage delay effect can be suppressed or eliminated. In an instance in which the above delay effect is suppressed by using thick-wall transparent electrodes as the scanning electrode and signal electrode, the transmittance in a light state is also lowered, resulting in a low contrast and low brightness of the picture.
Incidentally, as having been made clear in the U.S. Patents set out above, in embodying the surface-stabilized ferroelectric liquid crystal device, the space between substrates is required to be set with a distance small enough to suppress the specific spiral arrangement structure of a ferroelectric smectic liquid crystal and give a bistably oriented state, i.e., a distance of usually from 0.1 .mu.m to 3 .mu.m in approximation.
Experiments made by the present inventors revealed that the film thickness of a low-resistivity conductive film used for suppressing the applied-voltage delay effect, which is used when the surface-stabilized ferroelectric liquid crystal device is applied to a large screen display panel, may be brought into a thick-wall state of not less than 0.1 .mu.m, and preferably not less than 0.5 .mu.m, thereby making it possible to better preventing the lowering of display quality level, ascribable to the delay effect.
However, the wiring of the above thick-wall low-resistivity conductive film in contact with the transparent electrodes has brought about the problem that a danger of a short between the upper and lower substrates increases at the part such wiring is made. In a disply screen, existence of even only one shorted part can be found by a viewer, thus making a serious problem from the viewpoint of the display quality level.
In the ferroelectric liquid crystal device, rubbing is applied on the surface of the substrate so that liquid crystal molecules may be arranged in a given direction. In applying this rubbing, there has been the problem of a phenomenon that the protruded part of the low-resistivity conductive film causes peeling.
In color display using a liquid crystal display panel, a color filter for full colors that employs an integrated body of fine microfilters for R (red), G (green) and B (blue) is provided on the liquid crystal display panel so that the color display can be reproduced using light rays passed through the liquid crystal display panel capable of optical switching with the color filter.
Color filters used for such purpose are disclosed, for example, in Japanese Unexamined Patent Publications No. 57-16407, No. 57-74707, No. 60-129707, No. 62-212603 and No. 62-218902.
In the above prior art, however, the following problems have been involved from the viewpoints of fabrication processes and color reproducibility.
(1) In the process of forming a colored resin layer, exposure to light is carried out using a photomask, with alignment at the desired position. However, there is a limit in the alignment precision, and hence it often occurs that a gap is made between the color resin layer and a light-screening layer or that the colored resin layer is formed overlapping on the light-screening layer. This gap results in a lowering of contrast in the case of liquid crystal display devices. In the case of image pickup devices, on the other hand, a flare phenomenon is caused to make an image difficult to view. The overlapping between the light-screening layer and colored resin layer may also give cell gap irregularities in the case of liquid crystal display devices to cause orientation disturbance, resulting in a lowering of the display quality level. PA0 (2) In order to cure the colored resin in the process of forming the colored resin layer, the exposure is carried out from the surface. Hence, the photo-curing sufficiently takes place at the area near to the surface but insufficiently at the interface with the substrate, so that, at the time of developing, cracking and turning-up and peeling of patterns tend to occur, and the stability of the process can be by no means well satisfactory. PA0 (3) In instances in which a material having a high reflectance (as exemplified by a metal) is used in the light-screening layer when the surface exposure is made, a photosensitive colored resin film on the light-screening layer may be cured with a great influence of the light that reflects around as a result of the exposure, making it difficult to carry out development. PA0 (4) In the above prior art, all instances essentially require the operation of alignment between the formation of a color filter pattern and the formation of a non-light-transmissive film pattern. Hence, there has been the problem that the influence on the precision of this alignment makes it difficult to form between color filter patterns a non-light-transmissive film pattern free of any light-transmissive area and coincident in size. When the overlapping has occurred between these, there also has been the problem that a faulting (a difference in level) made on the color filter makes it difficult to form a color filter having a structural strength and an excellent flatness.
The above four points have been serious problems from the viewpoints of fabrication processes and color reproducibility.